Continuing improvements in circuit design has lead to smaller and smaller integrated circuits used in electronic parts and equipment. These integrated circuits can have dimensions less than 2 cm.sup.2 with pin counts over 100. It is increasingly important to test such parts accurately and at high speeds in order to keep up with the production rates of the parts.
Integrated circuits were first packaged in dual-in-line packages (DIPs), with leads coming out on either side forming two parallel rows. However, as integrated circuits became smaller and more complex, the number of leads necessary to access these circuits increased while the package size decreased.
A recent solution to this problem of putting a large number of leads on a small package are the ball grid array (BGA) and pin grid array (PGA) integrated circuit packages. These packages are generally flat and square. They have a large number of leads coming out of the bottom of the package, parallel to each other forming a grid array. The BGA and PGA packages can, for example, contain a pin count in the order of 143 in a 13.times.11 array with a 40 milli-inch (1.016 mm) pitch, or distance, between each ball or pin.
With these new smaller integrated circuit packages came the need for new testing techniques. Because the devices had leads coming straight out of the bottom of the package, in order to test the devices, the packages would need to be perpendicularly inserted into a test site which would make electrical contact with each of the leads for testing.
Current methods of testing these integrated circuit packages include a test site comprising a plurality of pogo pins within a series of holes in a cubical container, or socket, such as that described in U.S. Pat. No. 4,700,132 to Yarbrough et al., assigned to the assignee of the present invention and incorporated herein by reference. Pogo pins have contact heads which ride up and down on a tiny spring within the pogo pin housing. A cup-shaped contact head may be used to conform to the leads of the device-under-test (DUT). Each of the holes corresponds to and registers with the leads from a BGA or PGA device such that the leads come in contact with the pogo pins, providing the electrical connection to the printed circuit board (PCB) necessary for testing.
To test the integrated circuit, the device-under-test (DUT) is placed in a test fixture populated with pogo pins that will provide the contact between the device and the printed circuit board. The cubical pogo pin sockets are made of non-conductive plastic having a plurality of holes drilled within to house the pogo pins.
Problems arise with pogo pin testing when the pitch of the leads drops below 40 milli-inches (1.016 mm). At a 40 milli-inch pitch (1.016 mm), the manufacturability of the socket is at its limit due to thin walls between the holes and the tendency of the drill used to fabricate these holes to "walk" causing oversized or skewed holes. As the lead count on microprocessors increases leading to lower pitch BGA and PGA packages, the current method of testing using pogo pin sockets will not work, since the sockets can not be manufactured to these smaller specifications.
Another problem arises with the costs of manufacture and repair of pogo pins. Since each pogo pin includes moveable parts and springs, malfunctions arise when either the pin breaks or the hole in the socket becomes obstructed by dirt or other particles. Repair of a broken pogo pin is time consuming and costly. A broken pogo pin would result in inaccurate test results and mislabeling of the tested device as defective. Thus, a test fixture will sometimes reject a part as "bad" when in fact the part is good but did not make contact with the PCB either because of a broken pogo pin or an obstructed hole.
A solution to this problem would be to provide an inexpensive test fixture which eliminates the need for pogo pins to allow for testing of circuits with less than 40 milli-inch (1.016 mm) pitch, while still providing accurate and rapid testing of components. Another solution to these problems would be to provide a test fixture which is easily cleaned and not susceptible to collection of dirt which would lead to erroneous test results.
Thus, there is a need in the art for a method and apparatus for testing integrated circuits which eliminates the need for pogo pins to allow for testing of circuits with less than 40 milli-inch (1.016 mm) pitch, which is inexpensive, easily cleaned, and not susceptible to the collection of dirt.